End of ink detection apparatus are known in the art for printing units and more generally for ink-jet printing heads.
In particular U.S. Pat. No. 4,183,029 describes a detection apparatus showing the end of ink flow in the duct between the reservoir and the printing-head, due to an obstruction therein, caused by an air bubble or an impurity.
U.S. Pat. No. 5,051,759 describes an ink-end detector, in which two electrodes are fitted in the sponge located in the container and impregnated with ink. The electrodes are connected to a detector circuit that measures the electrical resistance of the ink placed between the electrodes. As known, as the ink is consumed, the sponge progressively empties from one end to the other where the electrodes are fitted.
However the ink resistance does not vary extensively until the level of the ink reaches the zone of the electrodes, then rapidly increases thereafter.
U.S. Pat. No. 5,136,305 describes a similar end of ink detector, whereby a thermistor is inserted into the sponge, in contact with the ink and receiving a d.c. supply. Measurement of the temperature increase of the thermistor represents the reduction in volume of ink in the sponge.
U.S. Pat. No. 5,162,817 describes an end of ink detector for an ink-jet printing head fed by a replaceable cartridge. The head comprises its own reservoir complete with an ink-impregnated sponge, onto which the cartridge can be coupled.
The detector comprises three electrodes fitted in the sponge and selected in pairs for detection with the head in the horizontal or the vertical position.
European Patent Application No. 440 110 describes another end of ink detector for another ink-jet printer whereby the container containing an ink-impregnated sponge can be replaced and connected to the head by means of a hollow needle penetrating the reservoir.
The reservoir has two electrodes, one fitted in the sponge, and the other inserted in the feed duct to the head. The detection circuit comprises a compensation resistance for the ink-viscosity according to the temperature.
All of the end of ink detector apparatus outlined above have one similar characteristic in common. i.e. that the ink reservoir contains a sponge, with the electrodes fitted therein close to the ink outlet hole. For that reason the end of ink signal obtained as a function of the variation in resistance of the ink between electrodes, is only generated when a small volume of ink remains in the reservoir, representing approximately 15% of the total, corresponding to a consumption of approximately 85% of the available volume.
This is due to the fact that the curve (FIG. 1) for ink resistance R.sub.k variation between electrodes, as a function of the ink consumption S% is essentially flat until the ink-consumption S% reaches approximately 85% of the total, as previously stated, before suddenly rising beyond that value.
Such apparatus are nevertheless not able to indicate continuous variations in the quantity of ink contained in the reservoir in an intermediate zone between minimum and maximum ink content.